LOS ANGELES –Sen. Elizabeth Warren (D., Mass.) assured unions she’ll be one of their strongest allies in Congress as they fight for everything from a higher minimum wage to a federal budget that preserves Social Security benefits.

In a fiery speech during the kickoff of the AFL-CIO’s quadrennial convention here Sunday, Ms. Warren said she has fought and lost her share of battles in Washington but is prepared to keep fighting–with organized labor at her side–on behalf of workers who are being ignored.

She took shots at Wall Street banks, saying they need stronger rules and tougher enforcement; trade deals that she said benefit corporations too much; the automatic government spending cuts known as the sequester, which she called “stupid”; and a Supreme Court she said could end up as a “wholly owned subsidiary of big business. ”

Ms. Warren delivered her speech on Sunday instead of Monday as originally scheduled; she is expected to return to Washington for the Senate debate and vote on authorizing military action against Syria.

“From tax policy to retirement security, the voices of hard-working people get drowned out by powerful industries and well-financed front groups,” she said, adding that organized labor has been there at every turn to fight on the behalf of the workers.

“The fight continues to rage, and the powerful interests continue to be guided by their age-old principle: ‘I’ve got mine, the rest of you are on your own,’” Ms. Warren said. “However steep our climb, I am proud to stand with you, to march with you, and to fight side-by-side with you,” she added, in remarks that were met with a standing ovation and rousing applause.

Organized labor will need support of lawmakers like Ms. Warren to revive a labor movement that AFL-CIO President Richard Trumka says is “in crisis” amid declining membership, stagnant wages, laws unions say make organizing too hard and budget battles in states that have stripped collective bargaining rights from public-sector workers. Unions are also hoping a congressional overhaul of immigration laws would produce more U.S. citizens who’d be more likely to join their ranks.

But Mr. Trumka knows that not all Democrats — and even fewer Republicans — are so firmly aligned with labor’s agenda as Ms. Warren is. When she exited the stage after standing hand-in-hand with Mr. Trumka, he turned to the crowd and said: “If we could only clone her and we had another 60 or 70 like her.”

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